Acquisitions, Dispositions and Mortgage Repayments - Acquisitions (Details) $ in Millions																	12 Months Ended
	Dec. 21, 2016 USD ($) ft²	Oct. 17, 2016 USD ($) ft²	Oct. 11, 2016 USD ($) ft² property	Sep. 26, 2016 USD ($) ft²	Sep. 12, 2016 USD ($) ft²	May 13, 2016 USD ($) ft²	Apr. 29, 2016 USD ($) ft²	Mar. 31, 2016 USD ($) ft²	Dec. 18, 2015 USD ($) ft²	Nov. 18, 2015 USD ($) ft²	Nov. 03, 2015 USD ($) ft²	Oct. 23, 2015 USD ($) ft²	Sep. 14, 2015 USD ($) ft²	Sep. 01, 2015 USD ($) ft²	Jun. 26, 2015 USD ($) ft²	Jan. 15, 2015 USD ($) ft²	Dec. 31, 2016 USD ($) ft² property	Dec. 31, 2015 USD ($) ft² property
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Purchase Price																		$ 190.1
Purchase Price Adjustments/(Credits)																		(5.3)
Mortgage Notes Payable Assumed																		(28.4)
Cash Consideration																		156.4
Real Estate																	$ 239.3	186.4
Other																	$ (1.2)	$ (1.6)
Square Footage | ft²																	583,011	504,249
Number of properties acquired | property																	10	8
Real estate acquisitions [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Purchase Price																	$ 241.9	$ 187.2
Purchase Price Adjustments/(Credits)																	(3.8)	(5.3)
Mortgage Notes Payable Assumed																	(13.2)	(28.4)
Cash Consideration																	224.9	153.5
Real Estate																		183.5
Other																		$ (1.6)
Square Footage | ft²																		504,249
Fair value adjustments on mortgage notes payable assumed																		$ 0.4
Real estate acquisitions [Member] | California [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired						May 13, 2016					Nov. 03, 2015					Jan. 15, 2015
Purchase Price						$ 20.0					$ 47.0					$ 39.3
Purchase Price Adjustments/(Credits)						(0.3)					(3.4)					(0.2)
Mortgage Notes Payable Assumed						(13.2)
Cash Consideration						6.5					43.6					39.1
Real Estate						20.4					44.2					39.2
Other						$ (0.7)					$ (0.6)					$ (0.1)
Square Footage | ft²						63,012					99,942					110,679
Fair value adjustments on mortgage notes payable assumed																	$ 0.8
Real estate acquisitions [Member] | Colorado [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired													Sep. 14, 2015
Purchase Price													$ 6.5
Purchase Price Adjustments/(Credits)													(0.3)
Cash Consideration													6.2
Real Estate													6.3
Other													$ (0.1)
Square Footage | ft²													47,508
Real estate acquisitions [Member] | Minnesota [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired	Dec. 21, 2016								Dec. 18, 2015
Purchase Price	$ 12.6								$ 16.0
Purchase Price Adjustments/(Credits)	(0.1)								0.1
Mortgage Notes Payable Assumed	0.0								(9.5)
Cash Consideration	12.5								6.6
Real Estate	11.3								16.0
Other	$ 1.2								$ 0.1
Square Footage | ft²	48,281								64,143
Real estate acquisitions [Member] | Washington [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired	Dec. 21, 2016	Oct. 17, 2016			Sep. 12, 2016		Apr. 29, 2016	Mar. 31, 2016		Nov. 18, 2015		Oct. 23, 2015		Sep. 01, 2015	Jun. 26, 2015
Purchase Price	$ 5.1	$ 9.8			$ 53.1		$ 21.6	$ 38.3		$ 27.6		$ 8.8		$ 28.0	$ 14.0
Purchase Price Adjustments/(Credits)	0.0	0.0			(0.1)		(2.8)	(0.6)		0.1		(1.3)		(0.2)	(0.1)
Mortgage Notes Payable Assumed	0.0	0.0			0.0		0.0	0.0						(9.4)	(9.5)
Cash Consideration	5.1	9.8			53.0		18.8	37.7		27.7		7.5		18.4	4.4
Real Estate	5.2	9.9			54.6		20.1	37.7		27.6		8.6		27.8	13.8
Other	$ (0.1)	$ (0.1)			$ (1.6)		$ (1.3)	$ 0.0		$ 0.1		$ (1.1)		$ 0.0	$ 0.1
Square Footage | ft²	20,740	29,753			87,462		46,637	69,712		60,437		33,169		52,813	35,558
Real estate acquisitions [Member] | DISTRICT OF COLUMBIA
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired				Sep. 26, 2016
Purchase Price				$ 45.2
Purchase Price Adjustments/(Credits)				(0.1)
Mortgage Notes Payable Assumed				0.0
Cash Consideration				45.1
Real Estate				43.7
Other				$ 1.4
Square Footage | ft²				103,783
Real estate acquisitions [Member] | Maryland [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired			Oct. 11, 2016
Purchase Price			$ 36.2
Purchase Price Adjustments/(Credits)			0.2
Mortgage Notes Payable Assumed			0.0
Cash Consideration			36.4
Real Estate			36.4
Other			$ 0.0
Square Footage | ft²			113,631
Number of properties acquired | property			2
Land acquisitions [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Purchase Price																		2.9
Cash Consideration																		2.9
Real Estate																		$ 2.9